AMD don't support anything beyond SSE2 on GSDX, normal C2D and C2Q support SSSE3, latest C2D and C2Q support SSE4.1, AMD support SSE3 that isn't used ( SSE3 not SSSE3 ) and Phenom 2 support SSE4a that is less instructions then SSE4.1 so you can't use GSDX SSSE3 or SSE4.1 with it.

i wouldn't go with i7, its expensive and little gain to no gain depending of what you use for, C2D 8400 3.0Ghz or C2Q 9400s 65w ( for future support 4 cores now not very used since there very few programs that use 4 cores ) would be my buy, i7 sucks and i5 will suck, maybe next gen will get better if they make it more energy wise or else intel will suck for a long time.

i will say the same, it still doesn't support 4 cores and you are already thinking of 8 threads, if it is a monster job making 4 threads sync, i would say 8 is not going to happen, at least not with GSdx hardware, 8 threads will probably bring lots of problems even if they work. For me i7 is just like P4 it sucks, 130w, it is a heater, really bad for overcloking, the system cost a arm and leg, improvement performence very little, there still not enough support for 4 cores it will take time so 8 threads it will be better if you freeze yourself and program to wake in a distant future.